Observation was carried out of 44 patients affected with pulmonary pathology during the course of AIDS, each of whom presented a severe respiratory insufficiency, admitted to the Clinic of Infectious Diseases between May 1987 and March 1989. In the patients suffering from their first respiratory infection, a lower mortality rate was observed (11/28, 39.2%) compared with the patients suffering from a second or successive infection (10/16, 62.5%). In sixteen cases, the etiological agent was Pneumocystis carinii while in 14 subjects it was impossible to perform bronchoscopy due to particular conditions of the respiratory apparatus and diagnosis was made according to CDC clinical criteria. Several parameters were furthermore evaluated (age, duration of the symptoms prior to admittance, LDH, PaO2, WBC) as potential prognostic indices; at the conclusion of the study, no statistically significant differences were found, however, between the group of survivors and the deceased. For the specific anti PCP therapy, a great variety of drugs were administered; among them, first choice was given to cotrimoxazole. In particularly critical patients, methylprednisolone was added. In 21 patients, a mechanical respiratory aid (C-PAP) was applied with favourable results in 16 of them.